In rockets, ramjets and other vehicles which fly through air and space, it is a continuing problem to protect them from the effects of heat, both that generated by the passage through air and that due to the thermal energy which is expended in propelling the vehicle. Ablative materials have been developed as an efficient means for cooling nose cones and combustors. Such materials by a combination of fusion and evaporation absorb and convey away heat from a surface.
The present invention is particularly concerned with the use of ablative materials as a thermal barrier on the inside of the case of a rocket or ramjet engine. Generally, such engine cases are cylindrical structures and the.ablative material lines the interior. See for instance, U.S. Pat. Nos. 3,001,362 to Runton and 3,122,883 to Terner. U.S. Pat. No. 4,118,928 to Lyles shows a two piece liner with an undulating clearance interface, to absorb stresses.
In many rocket applications the duration of firing is relatively short and a variety of different ablation materials and attachment schemes may be used. However, in ramjet applications as well as in some rocket applications, the duration of firing is long. During this period the thermal barriers are exposed to an oxidizing hot gas environment and substantial pressure and it is more difficult to provide an effective barrier. In the past, two basic types of combustor liners have been found effective: one is a tape-wrapped thermosetting polymer material, such as silica fiber impregnated with phenolic resin. But this has been prone to oxidation failure of the resin resulting in the hot combustion gases penetrating the liner and contacting the metal case. In addition, such materials also tend to have high thermal conductivities when they become charred; and this is disadvantageous insofar as insulating the metal case. The other type of liner is elastomer, such as the silicone base elastomer Dow Corning DC 93-104 (Dow Corning, Midland, Mich.). These materials are more desirable because they are oxidation resistant and have lower thermal conductivity when charred. However, because they are flexible during use they are subject to blistering if gases penetrate into the region between the elastomer liner and the metal case. When a liner blisters or becomes unbonded from the combustor case, it becomes subject to being blown off from the surface it is intended to protect which will result in failure of the engine.
While the elastomeric insulations are nonetheless preferred, they have other distinct phenomena which cause design difficulties insofar as keeping them functionally attached to a metal case. First, they tend to swell as they become heated and this dimension change can cause failure of the bond with the combustor wall. The very same swelling stresses also tend to cause the elastomer to blister or raise up from the case surface and to thus project into the path of the combustion gases. It is believed also that gases may be generated near the case by elastomer degradation and contribute to the aforementioned blistering. Second, charring of the elastomer surface nearest the hot combustion gases occurs as a natural and intended degradation of the ablative material. But, associated with the formation of the char is shrinkage which causes fissures in the insulating surface layer. If the fissures should propagate completely through the elastomer barrier they will allow the hot combustion gases to come in contact with, and fail, the case.
Based on the foregoing observations, various techniques have been explored to find ways to maintain the elastomer in contact with the case and avoid great cracking. One approach has been to apply metal honeycomb cell material to the inner surface of the case prior to applying the ablative material. The cells run radially inward and are filled with elastomer. Thus, the ablative material divided up into separate elements and cracks are prevented from propagating along the interior surface of the material. The welding of the honeycomb material to the case retains the individual pieces of ablative material on the surface. However, the honeycomb is costly and completely filling the individual cells presents technical problems. In addition, the cumulative cross sectional area of metal running radially disadvantageously increases the amount of heat that is transferred to the case.
As a result, there has been a continuing need for improved ways of retaining ablative materials on surfaces of combustors and other aerospace devices.